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US President Barack Obama on Saturday vowed to continue air strikes against Iraqi jihadists if needed to protect US diplomats and military advisers.

Speaking in his weekly address, Obama said that he had authorised the strikes in Iraq to protect US personnel serving in the northern city of Arbil. ”And, if necessary, that’s what we will continue to do,” he said.

In a speech explaining the decision to hold air strikes, Obama said OBAMA the Islamic State's (IS) advance in Iraq had been "more rapid" that expected by policymakers inside and outside Iraq.

He said the crisis would not be solved in weeks, and that the most important agenda for Iraq should be forming a unified government.

The first US bombings struck IS positions and at least one convoy of vehicles carrying militants west of Arbil.

Obama said he had also authorised a “humanitarian effort” to help displaced civilians trapped by jihadists on Mount Sinjar in northern Iraq. He said in his latest speech that the US and its allies were considering how to create a "safe corridor" for moving people from the mountain.

US jets struck jihadist positions in northern Iraq on Friday in a potential turning point in a two-month-old crisis after Obama warned of militants carrying out a "potential genocide" against displaced minorities.

Thousands of Yazidis, a Kurdish-speaking minority, fled their homes when militants attacked the town of Sinjar and many have since been stranded in the nearby mountain range with no food and water.

“The thousands - perhaps tens of thousands - of Iraqi men, women and children who fled to that mountain were starving and dying of thirst. The food and water we airdropped will help them survive,” Obama said.

“I’ve also approved targeted American air strikes to help Iraqi forces break the siege and rescue these families.”

Obama stressed that the US could not and should not intervene in every world crisis. But he said when innocent people face a massacre and the US has the ability to stop it, the nation should not look away.

As head of the US military, Obama said: “I will not allow the United States to be dragged into fighting another war in Iraq. American combat troops will not be returning to fight in Iraq, because there’s no American military solution to the larger crisis there.

“What we will do is continue our broader strategy in Iraq. We will protect our citizens," he said, adding that Iraq should push for reconciliation and to fight terrorism.

Obama's order for the first US air strikes in Iraq since the end of the US occupation in 2011 came after Islamic State militants made massive gains on the ground, taking control of Iraq's largest dam and threatening autonomous Kurdistan.

Pentagon spokesman Rear Admiral John Kirby, writing on Twitter, said two US jets from the aircraft carrier USS George H. W. Bush had hit a militant artillery piece that had been firing on Kurdish forces defending their capital, Arbil.

Watch: Barack Obama authorises air strikes to prevent Iraq ‘genocide'

The US air armada's first mission was to drop food and water to thousands of people who have been hiding from the Sunni militants in the barren Sinjar mountains for the past five days. Many of them are Yazidis, whom the Islamic State has branded "devil worshippers".

The Pentagon also said late on Friday that cargo planes escorted by combat jets made a second air drop of food and water to thos on Sinjar.

Obama also justified the limited air strikes because of the jihadist threat to Washington's Kurdish allies, following a lightning advance that saw them move within striking distance of Arbil.

Panic had begun to grip the Kurdish capital after Islamic State thrust into the Nineveh plains separating their main hub of Mosul and the autonomous territory over the past two days.

The Kurdish peshmerga, short of ammunition and stretched thin along a huge front, have been forced to retreat in the face of brazen jihadist assaults.

Their withdrawal from Iraq's Christian heartland on Wednesday and Thursday sparked a mass exodus and appeared to spur Western powers into action.

"Earlier this week, one Iraqi in the area cried to the world, 'There is no one coming to help'," said Obama in a television address to the nation. "Well, today America is coming to help. We can act carefully and responsibly to prevent a potential act of genocide."

Chaldean patriarch Louis Sako said Iraq's largest Christian town of Qaraqosh was emptied of its population on Thursday and estimated the number of Christians forced out at 100,000.

The capture of the Mosul dam was another setback for the defending peshmerga and gave jihadists a power of life and death over a huge swathe of land.

A Kurdish and a local official said jihadists took it over on Thursday. They warned that any "unscientific manipulation" could have disastrous consequences.

The air strikes came as Iraq's top Shiite cleric all but ordered Prime Minister Nouri al-Maliki to quit. Yesterday's weekly sermon by Grand Ayatollah Ali al-Sistani was his clearest call for Maliki to go.

Though his sermon, read out by an aide, did did not mention Maliki by name, Sistani said politicians who clung to posts were making a "grave mistake", and leaders must choose a prime minister to end the security crisis.

Meanwhile, US aviation authorities yesterday prohibited American airlines and other commercial carriers from flying over Iraq, saying hostilities there could threaten safety.